Reduce output message of dune_instance macros

Summary

This MR adds a helper function message_verbose() for printing messages directly to the CMakeOutput.log. This is used to reduce the verbose output of the dune_instance cmake macros

Details

When "hundreds" of files are generated for tests or explicit instantiation of templates it is not necessarily useful for the user to see this long list of files when configuring a dune module. But, for debugging and checking that all files are generated correctly, one might need this information sometimes. So, instead of printing it as a STATUS using the message() macro of cmake, it is printed in the VERBOSE mode. Since this mode is available from cmake 3.15 only, a helper macro is added to emulate this behaviour.

Closes #213 (closed)